The present invention relates generally to an arc furnace and more particularly to electrical instabilities which arise in an arc furnace during its operation.
The torn "stray arcing" has been used to describe this type of instability for some evidence seems to indicate that stray arcing may take place inside the furnace e.g. between the electrode and the furnace roof, or between other surfaces inside the furnace.
The present invention has application to DC and AC electric arc furnaces.
During the operation of a DC-arc furnace slag is displaced by the action of the arc from the molten slag layer onto the side walls and roof of the furnace. Hot dust particles and condensing vapours also adhere to the side walls and the roof. The slags are generally non-conductive, or poor conductors, in a cold state.
At elevated temperatures the insulating properties of slag, and in particular of slags which contain high percentages of certain oxides such as titanium dioxide, deteriorate. The resistivity of these slags can drop to such an extent that the material becomes electrically conductive. Consequently, inside the furnace, a conducting layer exists on the roof and side walls thereby imparting to the roof and side walls the same electrical potential as the top of the molten bath inside the furnace. The conducting layer thus promotes arcing for it provides a current path between cathode and anode.
The conditions inside the furnace, which give rise to stray arcing, are variable. For example the main arc is not perfectly stable, frothing and sparking take place, the slag is produced over a period of time, the level of the molten bath changes, and fluctuations exist in the rate, and the composition, of material feed to the furnace. Consequently measures which are taken to control stray arcing should, preferably, be adaptable in response to changes inside the furnace whether of the aforementioned kind or due to other factors such as temperature and pressure fluctuations, and in response to variations in the power supply to the furnace i.e. in the voltage applied to, and the current drawn by, the furnace.
The arcing can damage components of the roof, shell and hearth of the furnace and can lead to substantial reductions in furnace productivity. In water cooled furnaces the rupturing of water conduits by arcing can lead to water entering the furnace which can result in a powerful and damaging explosion.
The invention is concerned with improving the economic performance of an electric arc furnace by reducing the likelihood of arc damage to the furnace.